1. Field of the Invention
The present invention relates to a diffraction type optical pickup lens and an optical pickup apparatus using the same. In an optical pickup apparatus commonly usable for two or more kinds of optical recording media, in which light beams having respective wavelengths different from each other are employed as irradiation light for the optical recording media depending on the kinds of optical recording media, the diffraction type optical pickup lens can favorably focus these light beams onto their corresponding recording media.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using a single optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 650 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on a dual-wavelength beam type which uses two light beams having wavelengths different from each other as irradiation light.
In the case where the disc thickness or numerical aperture differs between the above-mentioned two kinds of optical recording media, however, it is necessary for such an optical pickup apparatus to have different focusing actions for the respective wavelengths of light for carrying out reproducing or recording.
For responding to such a requirement, a system in which two objective lenses having respective focusing actions different from each other are made exchangeable depending on optical recording media for carrying out reproducing or recording has been known. However, it complicates the structure of the optical pickup apparatus, while opposing demands for making it compact and lowering its cost.
As an apparatus satisfying such a requirement, one in which one surface of a convergent lens having aspheric surfaces on both sides is provided with a zonal diffraction grating centered at the optical axis (Japanese Unexamined Patent Publication No. 2000-81566). Since diffracted light on the same order is utilized as effective recording/reproducing light with respect to two wavelengths of light, this diffraction type lens does not function as a diffraction grating having a wavelength selectivity. Therefore, in the diffraction type lens disclosed in this publication, the degree of freedom in setting the focusing position may greatly decrease, so that there may be cases where two luminous fluxes having respective wavelengths different from each other are hard to converge at positions different from each other in practice.
On the other hand, a diffraction type lens in which two zone plates having a wavelength selectivity acting on only their corresponding wavelengths of light are formed on respective sides of a flat glass sheet on the light source side of an object lens has already been proposed in commonly-assigned applications Japanese Unexamined Patent Publication Nos. 2001-272516 and 2001-272517).
The commonly-assigned technique mentioned above is quite excellent in that two wavelengths of light can easily be focused onto optical recording media having respective NA and thickness values different from each other by using zone plates having a wavelength selectivity.
Since the flat sheet itself has no refracting power, however, it is necessary that a diffraction grating acting to converge a parallel luminous flux incident on the flat sheet bear all the focusing functions and aberration-correcting functions. Therefore, even when a certain degree of interval can be taken as a grating pitch in the vicinity of optical axis, the grating pitch may become smaller in the vicinity of marginal areas, whereby it is not so easy to process gratings, such as stepped gratings in particular. Though this may not be a very big problem if the technique for processing fine gratings improves as a matter of course, there is an urgent demand for finding measures against this problem at least at this point of time in view of the time and cost required for processing diffraction gratings.